Know when simple volts, amps and watts formulas apply

The relationship between voltage, current, resistance and power depends on the electrical model being used. A number produced by a simple calculator should not be treated as a cable, breaker or installation recommendation.

Work through a DC resistance example

An ideal 12 V DC supply across a 6 Ω resistor gives I = V ÷ R = 2 A. The resistor's power dissipation is P = V × I = 24 W. These values describe the assumed ideal circuit; they do not select a real component's rating or thermal arrangement.

Check power and current on the same basis

Dividing watts by volts is appropriate for a DC load under the assumed operating conditions. For a single-phase AC load, real power also depends on power factor when using RMS voltage and current. Three-phase relationships require additional assumptions. Do not use a simplified DC calculation to size an AC installation.

Distinguish labels from measurements

A device's input rating, output rating and actual operating consumption may differ. Motors, power supplies and variable loads may have behaviour that a single fixed wattage cannot describe. Use a suitable measured value or manufacturer specification and record what it represents.

Keep the result within its scope

The Job Totals tools provide formula calculations, not electrical design, fault protection or safe working instructions. Installation and circuit selection need the applicable design and qualified electrical advice. If the circuit type is unclear, resolve that first rather than treating a default voltage as confirmation.

Found a measurement or calculation issue? Send the example and your correction.